A major marketing advantage which plastic ophthalmic lenses currently enjoy over glass lenses is the capability of accepting, on their surfaces in the finished state, organic tints exhibiting a wide range of colors. That is, purchasers of plastic lenses have the option of selecting lenses custom tinted in a substantially infinite spectrum of colors and styles, and those lenses can be readily surface tinted by the eyeglass dispenser, e.g., by an optician, optometrist, or ophthalmologist, in his office or laboratory within a relatively short time. As an apparent corollary to that situation, the ready capability of surface tinting plastic lenses provides the eyeglass dispenser with the opportunity to add value to a finished lens with minimal cost and effort.
Numerous attempts have been made to apply surface tints to glass lenses utilizing organic dyes operable with plastic lenses. Unfortunately, those efforts to date have not provided tints of acceptable visible transmissions, i.e., less than 50% and, preferably, less than 30% transmission, where the coatings also satisfied five other practical requirements. Hence, the basic requisites for a tintable coating comprise: (1) it must be tintable with dyes operable with CR-39 plastic, CR-39 is a registered trademark of PPG Industries, Pittsburgh, Penna., referring to [poly(dially diglycol carbonate)], the material most commonly used commercially in the production of ophthalmic lenses, inasmuch as the eyeglass dispenser does not wish to stock two different sets of dyes; (2) it must demonstrate resistance to abrasion superior to that exhibited by CR-39 plastic; (3) it must be sufficiently adherent to glass to withstand boiling water to which it is exposed during the tinting operation, and to withstand surface abuse and environmental stresses encountered in normal wear; (4) it must be optically clear, i.e, essentially free from haze and other visual defects; and, from a practical point of view, (5) it must be relatively easy and inexpensive to apply. Coatings operable on glass surfaces would impart to glass lenses such capabilities as fashion tints, gradient tints, sunglass tints, and ultraviolet radiation blocking tints.
As was observed above, the search for a tintable polymer coating for use with glass ophthalmic lenses has been ongoing for a number of years. Various melamine, alkyd, and polyester resins have been investigated, some of which demonstrated greater affinity for CR-39 dyes than CR-39 itself. Unfortunately, those resins exhibited a lower resistance to abrasion than CR-39 and, hence, were deemed unacceptable on that score.
Based upon that previous work, we focussed our experimentation on resins which are acknowledged to manifest high abrasion resistance. Such resins fall into two general classes, viz., silicones and polyurethanes. Silicones employed as abrasion-resistant coatings typically consist of highly crosslinked alkyl siloxanes reinforced with colloidal silica copolymerized into the matrix. Thus, abrasion resistance is derived from a combination comprising a very rigid matrix and a hard filler. Two mechanisms are available in polyurethanes to impart scratch resistance. The first group generally relies on toughness. Those materials are also highly crosslinked and can be either air or moisture cured. The second group is composed of lightly crosslinked urethanes which exhibit cold-flow characteristics. The latter confer self-healing capabilities to the resin so that, although the resin can be scratched fairly easily, the scratches become less noticeable with time.
Several resins from each class were screened for chemical durability, hardness, and tintability. Chemical durability was evaluated as a set of pass/fail tests by exposing the coating to boiling water and to several such common household chemicals as acetone, isopropanol, and liquid detergent for about 30 minutes. Hardness was evaluated employing a pencil hardness method following Dow Corning Corporate Test Method CTM-0214. Tintability was evaluated qualitatively after immersion in BPI blue tint CR-39 dye for about five minutes at 95.degree. C. Those tests demonstrated the substantial superiority of silicones with respect to chemical durability and hardness, but they were somewhat inferior to the polyurethanes with respect to tintability. Because it was not believed that the chemical durability and hardness of a polyurethane could be improved to match that of the silicones, we sought means to enhance the tinting capability of the silicones.
Various silicone resins are currently employed commercially as scratch resistant coatings for plastic (CR-39) lenses. Unfortunately, when applied to glass surfaces, those coatings do not tint to acceptable levels with dyes operable with CR-39 plastic. The above-described screening tests indicated that the tinting capacity of the coating was actually much less than that of CR-39. Accordingly, it was conjectured that the acceptable tinting of the CR-39 lens was the result of the dye passing through the silicone and impregnating the CR-39 plastic. Because no like mechanism exists between the dye and glass, the level of tinting of glass lenses was insufficient.
Because silicone resins provide the other physical and chemical characteristics desired in coatings for ophthalmic lenses, the primary objective of the present invention was to develop means for improving the tintability of those resins by dyes operable with CR-39 plastics such as to render them acceptable for tinting glass lenses.